1. Technical Field
The present invention relates to a substrate for an electro-optical device, to a method of testing the same, to an electro-optical device, and to an electronic apparatus. More particularly, the present invention relates to a substrate for an electro-optical device having a plurality of pixels provided with a plurality of switching elements, to a method of testing the same, to an electro-optical device, and to an electronic apparatus.
2. Related Art
In general, display devices such as liquid crystal devices have been widely used in apparatuses such as cellular phones, projectors, or the like. The liquid crystal display devices using thin film transistors (TFTs) have a structure in which a TFT substrate and a counter substrate are bonded to each other and liquid crystal is inserted therebetween. A test for determining whether the manufactured liquid crystal device operates normally has been generally performed with respect to a finished product. For example, a predetermined image signal is input to the liquid crystal device as display data and projected and displayed, so that data is correctly displayed. However, in this case, it is checked whether defective pixels exist.
However, when adopting a method of performing the test for the finished product, there are cases in which defective products are discovered after a process of manufacturing the substrate. For this reason, the discovery of the defective products becomes delayed, which is not preferable in the management of the manufacturing process.
For example, a period until information about discovery of a defective product is fed back is lengthened at the time of the process management. As a result, a period for which a yield is lowered is lengthened, so that a manufacturing cost increases. Further, since a period until feedback is made is increased from an evaluation of a trial product to a design thereof even in the case of the trial product, a product development period is lengthened, which results in an increase in a product development cost. For this reason, after the product is finished, it is difficult to repair the defective product.
Accordingly, it has been required that a discovery of defective products, particularly, defective pixels of the display device are discovered in the process of manufacturing the substrate.
As one example of these test methods, a technology has been suggested in which a testing probe comes into contact with an electrode pad of a liquid crystal display device, a predetermined current is supplied thereto, and a test of the liquid crystal display device is performed (For example, JP-A-5-341302). Further, a technology has been suggested in which a predetermined voltage is applied to each pixel of a TFT substrate in accordance with a capacitance characteristic of the pixel, and a function of the TFT is tested on the basis of waveforms of a discharged current and a discharged voltage (for example, JP-A-7-333278).
Furthermore, a technology has been suggested in which an amount of changed potential of a pixel electrode is detected using a testing counter electrode corresponding to a pixel electrode of a TFT substrate, and an operation test of each pixel electrode is performed (for example, JP-A-10-104563).
In the technologies disclosed in JP-A-5-341302 and JP-A-10-104563, mechanical position precision is required in a test device such that a predetermined probe comes into contact with or comes close to the electrode pad from the outside of the substrate. As a result, there is a problem in that a detecting time is lengthened so as to obtain mechanical alignment precision. Further, in a case of testing a liquid crystal display device with high precision, since thin probes should be brought into contacts with a plurality of electrode pads through mechanical control, there are cases in which the above-mentioned technologies cannot be applied.
In addition, in general, the capacitanes of various capacitance components between the liquid crystal display device and a measurement device, for example, the capacitances in a source-line, an image signal line, and an electrode pad terminal are much larger than a capacitance of the pixel including an additional capacitance of the electrode. A voltage applied to the pixel electrode is determined according to a ratio between the capacitance of the source line and the capacitance of the pixel, and has a minute voltage level. For this reason, if the voltage held in the pixel is extracted from the electrode pad or the like, a noise having a large level overlaps the pixel potential having the minute level because of the capacitance of the source line, so that the measurement precision of the pixel holding voltage is extremely deteriorated, which results in insufficient measurement precision.